1. Field of the Invention
This invention relates to a method of correcting a read-out error of image signals read out by scanning a stimulable phosphor sheet carrying a radiation transmission image stored therein. This invention particularly relates to a method of correcting an image signal read-out error caused by distortion of scan lines in the forward main scanning direction and the backward main scanning direction when the stimulable phosphor sheet carrying a radiation transmission image stored therein is scanned by stimulating rays in the forward and backward main scanning directions by use of galvanometer mirrors and light emitted from the stimulable phosphor sheet in proportion to the stored radiation energy upon exposure to the stimulating rays is photoelectrically read out and converted into image signals.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the apparatus of FIG. 1, a radiation image stored in a stimulable phosphor sheet is read out by scanning the sheet with a laser beam as stimulating rays in the forward and backward main scanning directions. A laser beam 1a of a predetermined intensity emitted from a laser beam source 1 is slightly deflected by a subsidiary galvanometer mirror 2a into a laser beam 1b. By a main scanning galvanometer mirror 2b, the slightly deflected laser beam 2b is further deflected in the main scanning direction as indicated by the arrow A to form a laser beam 1c. Thus the laser beam 1c impinges upon a stimulable phosphor sheet 3 and scans it in the main scanning direction. In this case, the direction of slight deflection by the subsidiary galvanometer mirror 2a is normal to the direction of deflection by the main scanning galvanometer mirror 2b, i.e. to the main scanning direction. Namely, as a result of the slight deflection of the laser beam 1b by the subsidiary galvanometer mirror 2a, the laser beam 1c is slightly deflected in the direction as indicated by the arrow B with respect to the main scanning direction as indicated by the arrow A on the stimulable phosphor sheet 3. While the laser beam 1c impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed at a predetermined speed in the direction as indicated by the arrow B (sub-scanning direction), for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1c. As the stimulable phosphor sheet 3 is scanned by the laser beam 1c, the portion of the sheet 3 exposed to the laser beam 1c emits light having an intensity proportional to the radiation energy stored. The light emitted from the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its front end face 4a positioned close to the sheet 3 in parallel to the main scanning line. The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted from the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its front end face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5. Thus the light emitted from the stimulable phosphor sheet 3 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or stored in a magnetic tape 8.
FIG. 2 is a schematic view showing the scan lines of the laser beam scanning on the stimulable phosphor sheet in the apparatus of FIG. 1 and the waveforms of signals for driving the galvanometer mirrors.
In FIG. 2, solid lines L1, L2, L3, L4 and L5 designate scan lines drawn by a laser beam impinging upon the stimulable phosphor sheet 3 when only the main scanning galvanometer mirror 2b is activated without operating the subsidiary galvanometer mirror 2a. In this case, since the scan lines have a predetermined width (approximately 100 .mu.m), they overlap or separate from each other at their end portions and, as a result, the electric image signals obtained by scanning the stimulable phosphor sheet 3 with the laser beam become incorrect. Therefore, the aforesaid subsidiary galvanometer mirror 2a is used to scan so that the forward scan lines L1, L3 and L5 are parallel to the backward scan lines L2 and L4. For this purpose, the main scanning galvanometer mirror 2b is operated by a main scanning signal S0 as shown at the right end side of FIG. 2 to conduct scanning in the main scanning direction, and the subsidiary galvanometer mirror 2a is operated by a slight deflection signal S1. As a result, the forward scan lines L1, L3 and L5 are formed as shown in FIG. 2, and the backward scan lines L2 and L4 shift in the sub-scanning direction to form backward scan lines L2' and L4' as indicated by the dotted lines parallel to the forward scan lines L1, L3 and L5.
In this manner, a radiation image stored in a stimulable phosphor sheet can be read out in both forward and backward scan directions by scanning the stimulable phosphor sheet by a laser beam in the forward and backward main scanning directions. However, the rotation angle of the subsidiary galvanometer mirror 2a for slightly deflecting the laser beam is very small, and it is not always possible to control the rotation angle of the subsidiary galvanometer mirror 2a so that the rotation angle exactly conforms to the slight deflection signal S1 as shown at the right side of FIG. 2. As a result, the backward scan lines do not become parallel to the forward scan lines, but instead are partially distorted. Thus the forward scan lines and the backward scan lines partially overlap or separate from each other at both end portions of the scan lines as shown in FIG. 3B, and electric image signals obtained by reading out the radiation image stored in the stimulable phosphor sheet become incorrect.
FIG. 3B is a schematic view showing the case where forward scan lines and backward scan lines partially overlap or separate from each other due to an error in the slight deflection by the subsidiary galvanometer mirror 2a. In this case, in overlapping portions bi, bi+1, . . . , image read-out by scanning is conducted also for the portion where the stimulable phosphor of the stimulable phosphor sheet was stimulated and the radiation energy stored therein was erased by the previous scanning. In separating portions ci, ci+1, . . . , the radiation energy stored in the stimulable phosphor at these portions is not stimulated nor detected. Therefore, electric image signals obtained by scanning the stimulable phosphor sheet in this manner are incorrect, and an image reproduced by use of the electric image signals becomes incorrect or unsharp. Such a read-out error can be corrected and a correct electric image signal can be obtained by installing an optical error correcting means such as a light deflector in the laser beam path to prevent the forward scan lines and backward scan lines from overlapping and separating from each other. However, an optical error correcting means such as a light deflector has a complicated configuration so that when the optical error correcting means is incorporated in a radiation image read-out apparatus it is difficult or impossible to minimize the size of the radiation image read-out apparatus and to reduce the cost thereof.